The structures of chlorophyll and derivatives, and their numbering system, are shown in Scheme 1.

Over 20 years ago, the issue arose as to the nature of the self-assembled structures of the bacteriochlorophyll c molecules that comprise the intact chlorosomes of green photosynthetic bacteria (note that bacteriochlorophyll c is a chlorin, not a bacteriochlorin; this unfortunate misnomer is often a source of considerable confusion). Bacteriochlorophyll c differs from chlorophyll primarily in (1) having a 3-(α-hydroxyethyl) group in place of the 3-vinyl group, and (2) lacking the β-ketoester attached to the isocyclic ring. Smith, Kehres, and Fajer25 put forth a model for molecular interactions underlying aggregation. This model, which was developed in part by examination of the “aggregation” of bacteriochlorophyll c and analogues in hydrophobic organic solvents, with some embellishment is now widely accepted (Scheme 2).
In the early 1990s, Holzwarth began to address the question of what key substituents at the perimeter of the bacteriochlorophyll molecule are essential to cause the self-assembly process.26 Holzwarth in particular examined the R and S configurations of the α-hydroxyethyl unit at the 3-position (i.e., the 31-hydroxyethyl group) of the bacteriochlorophyll c molecule, because the natural system apparently uses a 2:1 ratio of the R:S mixture.

Two of Holzwarth's group members, Hitoshi Tamiaki and T. Silviu Balaban, continued this line of research. Tamiaki, employing synthetic derivatives of naturally occurring chlorophylls and bacteriochlorophylls, has examined the requirements for substituents at a variety of positions about the perimeter of the macrocycle on the self-assembly process. Balaban, on the other hand, has employed synthetic porphyrins (and just recently, some chlorins27) with different patterns of substituents and examined their self-assembly properties. Their findings can be summarized as follows.
Balaban's work, while unconstrained by starting with the naturally occurring tetrapyrrole macrocycles, has focused almost exclusively with the more synthetically accessible porphyrins rather than hydroporphyrins. He first showed that the α-hydroxyalkyl unit and keto groups must be directly attached to the porphyrin macrocycle, not at the p-positions of meso-aryl rings.28 He then showed that self-assembly occurs with zinc porphyrins bearing 10,20-bis(3,5-di-tert-butylphenyl) substituents and: (1) 31-hydroxyethyl and 13-acetyl, 15-formyl, or 17-acetyl groups; (2) 5-(hydroxymethyl) and 15-formyl or 13-acetyl groups; or (3) 31-hydroxyethyl and 17-acetyl groups. A representative structure is shown in Scheme 3. This work suggested that a linear arrangement of interacting groups is not essential, although the self-assembly is more extensive in the compounds having such a linear configuration.27 In all of Balaban's studies, the key observable for the occurrence of self-assembly was the alteration of the optical absorption spectrum. No studies of energy transfer were performed. Moreover, it is noteworthy that the porphyrin aggregates did not exhibit the type of relatively sharp, banded spectra characteristic of the naturally occurring hydroporphyrins.

Within the confines of the scientific box presented by using naturally occurring (bacterio)chlorophylls as synthetic starting materials, Tamiaki has extensively explored the effects of substituents. (In general, the naturally occurring tetrapyrroles bear a full or nearly full complement of substituents about the perimeter of the macrocycle, thus not giving free rein to studies where substituents are employed to tune oxidation potentials and photophysical properties, as has been done with synthetic porphyrins.) For the most part, this work was done using chlorins rather than bacteriochlorins. This work spans ˜50 papers over a 13-year period. I have attempted to summarize Tamiaki's pertinent findings in Table 1. The structures employed generally are zinc chelates of naturally occurring chlorins (see the generic structure in Scheme 4), not bacteriochlorins. The assemblies occur upon placing the compounds in a hydrophobic organic solvent.
The studies show that the self-assembly process requires an α-hydroxyalkyl group, a keto group, and a metal with an apical binding site, preferably in an essentially linear arrangement, and preferably without a 132-carbomethoxy group. All of the studies relied on absorption spectroscopy (UV-Vis, IR) to assess the existence of oligomerization. A few studies of energy transfer in self-assembled systems derived from the zinc chelate of the chlorin bacteriochlorophyll c were performed, suggesting that the assembled structures support energy transfer.41
TABLE 1Summary of Tamiaki studies of substituted chlorins.Scheme 4. Core chlorin in Tamiaki's studies (left) and bacteriochlorophyll d (right) RefAnalogueVariableResult29Zn Me Bph d31R versus 31S epimerQy maximum of the assemblies differ by 12 nm30Zn Me Bph dReplacement of the 31-hydroxy group withAmines cause presence primarily of dimersan amino group (NMe2, NHMe, NH2)(perhaps antiparallel) rather than the typicalextended aggregates (parallel)31Zn Me Bph clength of the esterified alkylNo effect for 1-4 isoprene unitsgroup at the 173-position32Zn Me Bph aZinc tetrapyrrolic macrocycles (porphyrin,aggregates occur regardless of the porphyrinicchlorin, or bateriochlorinhydrogenation state33Zn Me Bph dA 131-hydroxy (rather than keto) groupAlso results in self-assembly34Zn Me Bph dSize of alkyl group at 3-position where theThe relative stability of aggregates decreaseshydroxy group is at the 31-positionalong the series of 3-substituents:hydroxymethyl > R-α-hydroxyethyl~α-hydroxy-α-methylethyl > S-α-hydroxyethyl35Me Bph delectron-withdrawing groups (cyano,redshifts of the Qy band of up to 30-nmcarbomethoxy) at the 32-vinyl group36chlorophyll α132-epimer (where the 13-carbomethoxyaffords a different aggregate than the naturallygroup is on the same plane as the 17occurring chlorophyll αalkyl substituent)37Zn Me Bph dInverse keto and hydroxyl groupsspectral shifts comparable to that of the normal(i.e., 3-acetyl, 131-hydroxy)substituent pattern (13-keto, 31-hydroxyethyl)38Zn Me Bph d131 R versus S epimer131 R epimer gives a face-to-face closed dimer;131 S epimer gives head-to-tail open oligomers39Zn Me Bph d71-hydroxymethyl group (3-vinyl) versus71-hydroxymethyl group (3-vinyl) causes self-normal 31-hydroxymethyl group (7-assembly but the spectral shift (1320 cm31 1)methyl)was less than normal (1860-1940 cm−1)40Zn Me Bph d8-hydroxymethyl group versus normalcauses self-assembly as long as the 3-(31-hydroxymethyl and 8-alkyl groups)substituent is ethyl rather than acetyl, theaggregates are less structured than the normalcompound41Zn Me Bph csubstituents at the 20-positionAll are tolerated in the self-assembly process(H, F, Cl, Br, Me, CF3)42Zn Me Bph dbulky ROCH2— groups at the 7-position;All are tolerated in the self-assembly processR = Me, acetyl, or pivaloyl43Zn Me Bph dpresence of a methoxycarbonyl group atdoes not preclude self-assembly, but the extentthe 132-position (resembling chlorophyllextent of assembly is diminishedα)44Zn Me Bph dElongation of the 3-hydroxyalkyl chaindoes not cause cessation of self-assembly butalong the series 1-hydroxyethyl,the extent of redshift of the Qy band is2-hydroxyethyl, 3-hydroxypropyl,diminished with increasing lengthand 3-hydroxyprop-1-enyl45Zn Me Bph ddivalent metals Mg, Zn, Cd, and Coassembly45Zn Me Bph ddivalent metals Ni, Cu, Pd, or Agno assembly45Zn Me Bph dtrivalent metals Fe(III) and Mn(III)partial assembly46Zn Me Bph dEthenylation of the 8-position (vinyl,noticeable redshifts of the B band but not thestyryl)Qy band; no effects on assembly process47Zn Me Bph dMove the formyl group from the 7-causes a 22-nm redshift of the Qy band.position (as in Bchl e) to the 8-The self-assembled aggregate of the latter alsopositiongave a more extensive redshift than the former(In Table 1, Zn Me Bph d refers to the bacteriochlorophll d analogue where zinc is the central metal and R is methyl (on the ester). The variable refers to subsequent modifications from this core motif)
The study of light-harvesting in the assemblies described in this work has been quite limited despite the elegant work of Tamiaki and others. The limitations are as follows:                (1) self-assembly has hardly been studied with bacteriochlorins (i.e., tetrahydroporphyrins) regardless of their source and certainly not with stable bacteriochlorins.        (2) assessment of oligomerization has largely been inferred on the basis of absorption spectroscopy.        (3) of the assemblies that have been prepared, few energy-transfer studies have been performed, even fewer of which included time-resolved measurements. To our knowledge, no estimates of exciton diffusion lengths, for example, have been made in any such materials.        